If I understand it right you need to convey at the same time/place the
bad/good judgment of a value and a tendency 'getting worse'/'getting
better', but not as an estimation of a trend, but considering only
comparison with the past.
Especially considering only comparison with the past appears to me to be
hard to visualize and I have no slightest idea how to convey it, so I
will just neglect this requirement replacing it with a common sense
understanding of a trend as an estimation of future development (i.e.
estimation of what is expected to come) like you do yourself mentioning
'getting worse'/'getting better' which terms don't convey in my
understanding the fact that only the past is considered (in case a node
represents an instant of time in the past and its future state is also
If your current visualization utilizes the colors used in traffic lights
maybe you can use the common sense of traffic light logic known in case
when the lights are also equipped with a yellow lamp showing the coming
change and extend it according to your needs?
Another option popping up in my mind is usage of smileys on a sunny or
rainy background (happy smiley on a rainy background showing "ok, but
it's getting worse", happy smiley on sunny background showing "ok and
getting even better", etc.).
If all of the 'iconizing' won't fulfill the expectations as the audience
fails to interpret intuitively the visualization the right way, you have
still the option to provide the information just as a text ('getting
worse'/'getting better') being free to provide a graphical visualization
along with the text until the audience becomes 'conditioned' to it and
will interpret it always as intended.